In recent years, antenna duplexers have been widely used that separate transmitting waves and receiving waves using an elastic wave filter device in the antenna circuit of a mobile phone. FIG. 11 is a circuit diagram of a conventional elastic wave filter device. The device includes panel 1 made of a ceramic substrate and chip 2 attached onto panel 1. Panel 1 includes common terminal 3, transmission-side terminal 4, receiving-side terminal 5, ground electrode 6, and wirings connecting these terminals. Chip 2 made of a single-crystal piezoelectric substrate, includes, on its surface, first ladder-type elastic wave filter 7 forming the transmission-side filter; and second ladder-type elastic wave filter 8 and longitudinally coupled resonator-type elastic wave filter 9 both forming the receiving-side filter. Filters 7, 8, and 9 are formed of elastic wave elements (not shown).
First ladder-type elastic wave filter 7 includes series arm resonator 10 and parallel arm resonator 11. One end of filter 7 is connected to common terminal 3, and the other end is connected to transmission-side terminal 4. The ground-side wiring of parallel arm resonator 11 is connected to ground electrode 6.
Second ladder-type elastic wave filter 8 and longitudinally coupled resonator-type elastic wave filter 9 are serially connected to between common terminal 3 and receiving-side terminal 5. Filter 8 includes series arm resonator 12 and parallel arm resonator 13, and the ground-side wiring of resonator 13 is connected to ground electrode 6. Filter 9 includes input IDT (inter-digital transducer) 15 and output IDT 16 between reflectors 14. Each one end of input IDT 15 and output IDT 16 is connected to the signal wiring, and the other end is connected to ground electrode 6.
The above elastic wave filter device is used as a high-frequency filter with its passband center frequency between around 800 MHz to 2 GHz in mobile communication equipment. One of the important required characteristics in this case is to reduce unnecessary signals; increasing the attenuation is demanded at frequencies out of the passband.
However, even if the electrode finger pattern of an IDT is optimized to increase attenuation of the elastic wave element itself out of the band, an elastic wave filter device that is a product finished by containing the elastic wave elements into a package (not shown) may exhibit degradation in out-of-band attenuation. Reasons for the problem include an electrode wiring pattern inside the package and presence of electric resistance in the conductive material electrically connecting the package with the elastic wave element. The other reason is various types of parasitic components generated (e.g. presence of parasitic inductance and floating capacitance, electromagnetic coupling between input IDT 15 and output IDT 16; and between input IDT 15 or output IDT 16 and ground electrode 6). Consequently, an elastic wave filter device produced by packaging elastic wave elements often exhibits high-frequency characteristics lower than those achieved by a single elastic wave element. Further, out-of-band noise components released from parallel arm resonator 11 of first ladder-type elastic wave filter 7 to ground electrode 6 sometimes flow from ground electrode 6 into the signal wiring through input IDT 15 or output IDT 16 of filter 9, degrading the attenuation characteristics.
There is known patent literature 1 for example as information on prior art documents related to the invention of the application.